The present invention relates to the uniform charging of randomly charged materials, in general, and to the neutralization of random bounded and free electrostatic charges on a moving web of such materials, in particular.
Random bounded and/or free electrostatic charges on a web can produce a number of product quality damaging web coating problems. Bounded charges are sometimes referred to as polarization or polar charges whereas free charges are commonly referred to as surface charges. In the photographic industry, for example, a significant non-uniform thickness distribution of a photographic coating material often results when such material is applied to a randomly charged web. Because of the high surface resistivity of present day high dielectric materials such as polyester based materials and the like employed in photographic film, it is fairly common to have relatively high polarization and surface charge levels, of varying intensity and polarity, occupying web areas closely adjacent one another. The use of such coating materials as a component of a photographic positive or negative, for example, often requires the use of relatively thick coatings to provide at least a minimum thickness coating throughout the web and thereby compensate for such non-uniform thickness distribution which necessarily results in an increase in the use of relatively costly photographic coating materials in order to produce an effective coating thickness. Visual effects such as photographic mottle are also a consequence of coating non-uniformly charged webs with photographic coating materials. Past practices included either tolerating this non-uniform charge distribution and its attendant disadvantages or attempting to neutralize a randomly charged web as much as possible prior to applying the photogrphic coating materials.
One technique described in U.S. Pat. No. 2,952,559 to NADEAU, involves passing a charged web between a pair of opposed grounded pressure rollers that are spring-force biased against opposite web surfaces for the purpose of neutralizing bounded or polarization-type electrostatic charges and then blowing ionized air onto surfaces of the web to first neutralize surface charges and then establish a particular web surface charge level prior to coating same. This resulting surface charge level is compensated for by applying a voltage to the coating applicator during the actual coating process having a polarity that is opposite to that of the web surface charge.
Another technique desribed in U.S. Pat. No. 3,730,753 to KERR involves "flooding" a web surface with charged particles of a first polarity so as to generally uniformly charge the surface and thereafter removing the charge imparted to said web surface so as to leave the surface generally free of charge. The amount of charge added to and/or the amount of charge removed from the web surface may be so controlled that the charge variation and the net charge on the surface is lowered to an acceptable low level.
While blowing ionized air onto a charged web surface or "flooding" same with charged particles are effective techniques for neutralizing unbounded or free surface charges they have a very limited effect on the control or neutralization of bounded or polarization charges. Neutralization of bounded or polarization charges in materials having relatively low surface resistivity (less than 1.times.10.sup.13 ohms.sup.2 per square) such as paper materials and the like, with a pair of opposed grounded pressure rollers pressing on opposite sides of the paper materials as shown in the above-cited NADEUA patent, may be effectively accomplished. However, when opposed pressure rollers are used with relatively high-dielectric materials such as polyester based materials and the like, the lowest polarization-type electrostatic charge level obtainable with such apparatus is in the range of 500-800 volts.
The primary object of the present invention is, therefore, to provide a method and apparatus for uniformly charging randomly charged materials having relatively high surface resistivity.
Another object of the present invention is to provide a method and apparatus for neutralizing bounded and unbounded random electrostatic charges on a randomly charged web.
A further object of the present invention is to provide a method and apparatus for establishing a uniform positive, negative or neutral electrostatic charge level on a randomly charged moving web.
Other objects, features and advantages of the present invention will be readily apparent from the following description of the preferred embodiment thereof taken in conjunction with the accompanying drawings.